It is well known that maleic anhydride can be produced by oxidizing unsaturated hydrocarbons having four or more of carbon atoms, such as butene, butadiene, cyclopentadiene, etc., with an oxygen-containing gas in the presence of a catalyst comprising vanadium pentoxide and phosphorus pentoxide (see Japanese Pat. Pub. No. 7888/1965 and U.S. Pat. No. 3,156,707).
It is also known in the art to produce maleic anhydride by oxidizing unsaturated hydrocarbons having four or more of carbon atoms in a gaseous phase by using a catalyst comprising vanadium pentoxide, phosphorus pentoxide, copper and niobium compound (U.S. Pat. No. 3,255,212), or a catalyst prepared by adding a slight amount of a compound of Group Ia elements of the Periodic Table to vanadium pentoxide and phosphorus pentoxide (U.S. Pat. No. 3,366,648), or a catalyst prepared by adding a compound of Group Ia elements of the Periodic Table to vanadium pentoxide, phosphorus pentoxide and copper (U.S. Pat. No. 3,385,796). There are also known a method using a catalyst prepared by adding iron, cobalt or nickel to a vanadium-phosphorus-oxygen type catalyst (U.S. Pat. No. 3,156,705), a method using a catalyst prepared by adding various kinds of metals such as alkali metals, iron, etc., as phosphorus stabilizer to a vanadium-phosphorus-oxygen type catalyst (U.S. Pat. No. 3,156,706), and a method using a catalyst containing an oxide of a metal selected from the group consisting of copper, silver, chromium, manganese, iron and cobalt in addition to vanadium pentoxide, phosphorus pentoxide and tungsten oxide (U.S. Pat. No. 3,478,063).
With a view to providing an industrially advantageous method for production of maleic anhydride, the present inventors have pursued various researches and experiments by adding various kinds of additives to the catalysts comprising vanadium and phosphorus oxides to determine the effects of such additives to the catalyst activity and revealed the fact that a significant improvement of the activity of the vanadium oxide - phosphorus oxide catalysts, can be attained when a specific amount of a potassium compound is added, and such finding has been applied for a patent (Japanese Pat. Pub. No. 22326/1970). The present inventors have made further studies for obtaining a still higher improvement of the catalyst activity and finally reached a finding that further addition of a specific amount of an iron compound to the above-mentioned types of catalysts can provide a more prominent improvement of the catalyst activity. This invention was attained on the basis of such finding.